1. Field of Invention
The present invention generally relates to the art of electrochemical cells. More particularly, the present invention is directed to a cell construction that makes use of unbalanced electrode components comprising one of the electrodes of an electrochemical cell, i.e., comprising either the anode or the cathode, to provide for end-of-life (EOL) or constant state-of-discharge indication.
In the present invention, either the anode or the cathode is characterized by an unbalanced structure provided by at least two dissimilar electrode components or electrode portions which each electrochemically react with the other electrode to provide electrochemical energy. The unbalanced electrode components can have dissimilar physical dimensions or dissimilar percent loadings of electrode active material. This renders the one dissimilar electrode component having the lesser physical dimension, i.e., the lesser surface area or lesser thickness, or having the lesser quantity of electrode active material, electrochemically unreactive prior to that of the greater of the dissimilar electrode components during the course of cell discharge. Upon the early exhaustion or unreactiveness of the lesser dissimilar electrode component, the total cell interelectrode working area is reduced by a predetermined factor. The remaining functional electrode component, i.e., the greater dissimilar electrode component provides the cell with sufficient capacity for electrical discharge at a predetermined lower energy level. The onset of reduced energy output acts as an end-of-life indication.
2. Prior Art
Battery powered implantable medical devices require some means of end-of-life indication for the battery in order that physicians have sufficient notice, or elective replacement indication (ERI), to complete replacement of the medical device and/or power source prior to system failure brought about by battery power depletion. Examples of such battery powered implantable devices are all types of pacemakers, implantable cardiac defibrillators, neurostimulators, drug delivery devices, and the like.
One method of providing end-of-life indication is through a specific electrochemical system. Certain cell systems have inherent electrochemical characteristics which are modified predictably with the progress of electrical discharge. One example is an alkali metal-halogen cell. Such a cell is described in U.S. Pat. No. 4,132,836 to Greatbatch and comprises an alkali metal anode, preferably lithium, a solid alkali metal-halogen electrolyte and a cathode comprising a mixture of two halogens, for example iodine and bromine. The two halogens provide for discharge of the cell at two different levels of cell output voltage thereby providing a two step output voltage-time characteristic.
Another example of an electrochemical system that is useful for providing end-of-life indication is an alkali metal-mixed metal oxide cell. Such a cell is described in U.S. Pat. No. 4,310,609 to Liang et al., which is assigned to the assignee of the present invention, and preferably comprises a lithium anode and a silver vanadium oxide cathode. The background or open-circuit voltage and the state-of-charge or expanded cell capacity of this system are well characterized functions of the depth-of-discharge of the cell. While this electrochemical system provides high energy density, the continuously sloping discharge curve makes Li/SVO cells impracticable as power sources for certain electronics that require a tighter output window.
It is also known to use other techniques in lieu of electrochemical means to demonstrate end-of-life indication for an electrochemical cell. In that respect, various electrode constructions have been described in the art for indicating state-of-discharge of an electrochemical cell and, subsequently, to predict end-of-life of the cell. A representative cell design is described in U.S. Pat. No. 4,563,401 to Kane et al., which comprises two anode-functioning members, one fitted inside the other. The anode members have cylindrical shapes with non-uniform lengths to provide a greater thickness where the anode members are coextensive. A cylindrical cathode is provided radially inside the anodes and during discharge, the thinner anode structure is depleted first.
This patent also describes a cell having two anode structures electronically coupled in parallel and having different thicknesses. The cathode material is placed at an intermediate location between the anode structures and both electrodes are separated from the casing by separator material. The present invention is preferably constructed in a case-negative or a case-positive configuration and both the anode and the cathode can be provided by the dissimilar internal electrode components.